The present invention relates to methods, systems and apparatus for transmitting data in mobile telecommunication systems.
Third and fourth generation mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architecture are able to support more sophisticated services than simple voice and messaging services offered by previous generations of mobile telecommunication systems.
For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as mobile video streaming and mobile video conferencing that would previously only have been available via a fixed line data connection. The demand to deploy third and fourth generation networks is therefore strong and the coverage area of these networks, i.e. geographic locations where access to the networks is possible, is expected to increase rapidly.
To help improve the performance and flexibility of wireless (“mobile”) communications systems, relay nodes are proposed to supplement transmissions associated with base stations. Examples of potential performance improvements associated with the use of relay nodes include:                increased coverage of high data rates within a cell        the provision of coverage to new areas        additional features such as temporary network deployments and group mobility.        
A relay node maybe considered as being distinct from a conventional cellular base station (e.g. a so-called eNodeB (eNB) in 3GPP LTE nomenclature) in the sense that a relay node is wirelessly connected to the RAN (radio access network) through a conventional base station. The base station through which a relay node connects to the RAN is often referred to a donor base station/donor eNB (i.e. the term donor base station may be used to refer to a base station serving a relay node). The radio network cell supported by the donor eNB in which the relay node is located may similarly be referred a donor cell for the relay node.
Relay nodes may in some respects be categorised according the type of wireless spectrum used for backhaul communications with the donor eNB. For example, “in-band” relay nodes communicate wirelessly with their donor eNB in the same spectrum as is used for communications with terminal devices/user equipment (UEs) within the donor cell, while “out-band” relay nodes communicate wirelessly with their donor eNB using different spectral resources from those used to communicate with UEs in the donor cell.
From a UE's perspective, relay nodes can also be classified into transparent and non-transparent types. For a transparent relay node the UE is unaware that it is communicating with the donor eNB via the relay node. For a non-transparent relay node, on the other hand, the UE is aware that is communicating with the relay node, i.e. the relay node presents to the UE as a conventional eNB.
From the perspective of the donor eNB, a relay node can be considered as simply a functional extension of the donor eNB, in which case the relay node will not have a cell identity of its own and its resources will be controlled by the donor eNB (i.e. at least one part of the Radio Resource Management (RRM) aspect of the Relay Node is controlled by the donor eNB). Alternatively, in some cases a relay node may be considered as serving an independent cell in its own right, in which case the relay node will have its own physical layer cell identity and ownership of the full RRM (i.e. the donor cell does not have control over the relay node's resources).
Relay nodes can also be distinguished from wireless repeaters. The function of a wireless repeater is simply to boost the power of the signals it receives. Wireless repeaters do not discriminate between wanted signal, interference or noise, and will re-broadcast all signal components received. Relay nodes, on the other hand, are regenerative repeaters in the sense that they decode a received signal and selectively re-broadcast appropriate components. Relay nodes can therefore provide for improved signal-to-noise ratios (SNRs), provided a signal is received at the relay node with a sufficient SNR to allow the relay node to decode it successfully.
Thus the deployment of relay nodes in a wireless telecommunications system can provide various different benefits as discussed above. Furthermore, specific deployment scenarios can be designed to promote certain benefits over others. For example, in a deployment where low-cost roll-out is important, an operator may choose to deploy relay nodes to increase the coverage area of each cell without the need to install fully commissioned base stations (including wired backhaul links). In another example, an operator may wish to improve the data rate available to users located in a certain portion of the cell. Installing a relay node can help to achieve this without increasing the total footprint of the cell.
Turning now to some of the types of services that are becoming more common with newer generation telecommunication networks, one such category of services is the Multimedia Broadcast/Multicast Services (MBMS) (see, for example, ETSI TS 122 246 [1]). MBMS services can, for example, include the following user services, as set out in Section 4 of ETSI TS 122 246 [1]:                Streaming services        File download services        Carousel services        Television services        
Data transported using the MBMS system is preferentially delivered either via broadcast or multi-cast techniques which allows the same content to be delivered to multiple users simultaneously, thereby saving resources in the physical layer of the radio and wired connections. Point-to-point delivery (unicast) methods may, however, by used in a case where service uptake is low.
The radio physical layer can include a technique for improving the efficiency with which broadcast/multicast services, such as MBMS, can be delivered. This enhancement is referred to as MBMS over a Single Frequency Network (MBSFN). With MBSFN the same waveform is transmitted from all base stations in a defined area at the same time. UEs are then able to combine all signals received no matter which base station they originated from, provided the signals arrive at the UE within a certain time window (signals from different base stations will be received at different times according to how far away they are). A major benefit of MBSFN transmission is an increase in the signal to interference plus noise ratio (SINR) at cell boundaries. The SINR improvement can be obtained because signals from neighbouring base stations are no longer a source of interference, but are an additional source of the wanted signal.
MBMS services are generally designed to be available to all terminal throughout a coverage area. Because of this MBMS data rates will typically be limited by cell-edge performance. Accordingly, when MBSFN transmission is used in the radio physical layer, the system can offer an overall higher throughput throughout the network because the performance at cell boundaries can be improved. This increase in throughput can in turn translate into improved service offerings (e.g. higher definition video, more TV channels, etc.).
However, for MBSFN to be effective, the typical inter-site base station separations distance must be sufficiently small to ensure the single frequency network (SFN) combining can occur at the cell boundaries to provide an improved SINR geometry. If the cells are spaced too sparsely, the system will become noise-limited at the cell boundaries resulting in either service outages or a reduced service capacity across the whole broadcast area, both of which are undesirable.
For operators that wish to deploy MBMS services using MBSFN transmission, relay nodes can offer a cost-effective deployment model for reducing the effective inter-site base station separation. This is because rather than deploying additional fully-equipped base stations, relay nodes could instead be used to enhance signal coverage at the cell boundaries. The operator could then offer an improved level of MBMS service with lower infrastructure cost coming from the lower equipment cost and absence of a separate backhaul connection for the relay nodes.
In view of the above there is a general drive towards the use of relay nodes for transmitting data intended for multiple recipients/UEs in wireless telecommunications systems, for example data associated with MBMS services, and doing so with improved efficiency.